The invention relates generally to particulate collectors using electrostatic forces, and more particularly to discharge electrode systems for use in an electrostatic precipitator.
Electrostatic precipitators (ESPs) are devices used to collect particles from gas streams, such as the gas streams from electric power plants burning coal. Charging electrodes (also called “discharge electrodes”) are critical components used in ESPs. Examples of such devices are shown in U.S. Pat. No. 6,231,643 to Pasic, et al., U.S. Pat. No. 7,976,616 to Alam, United States Patent Application Publication No. US2011/0056376 published Mar. 10, 2011, and United States Patent Application Publication No. US2012/0227588 published Sep. 13, 2012, all of which are incorporated herein by reference.
In a typical conventional ESP 2, shown in FIG. 9, vertical wire electrodes 4 are placed in the midsection of a channel formed between vertical parallel collector substrates 6. The most basic ESP contains a row of wires followed by a stack of spaced, planar metal plates. The heavy, typically steel, plates 6 are suspended from a support structure that is anchored to an external framework. Commonly, ten of the single precipitation channels constitute a single field. Industrial precipitators have three or more fields in series. An example of such a structure is shown and described in U.S. Pat. Nos. 4,276,056, 4,321,067, 4,239,514, 4,058,377, and 4,035,886, which are incorporated herein by reference.
A high-voltage DC power supply, typically of about 50 kV, is applied by a high voltage power supply 8 disposed electrically between the wire discharge electrodes 4 and the grounded substrate collector plates 6 (also called “collecting electrodes”), inducing a corona discharge between them. This transfers electrons from the plates to the wires, developing a negative charge of thousands of volts on the wires relative to the collection plates. In a typical ESP, the collection plates are grounded, but it is possible to reverse the polarity.
The gas stream and particles flow through the spaces between the wires, and then pass through the rows of plates. During this flow, the gases are ionized by the charging electrode, forming a corona. As particles are carried through the ionized gases, the particles become negatively charged. A fraction of ions, which migrate from the wires towards the plates, attach to the dust particles in the exhaust gas flowing between the plates 6. When the negatively charged particles move past the grounded collection plates, the strong attraction causes the particles to be drawn toward the plates until there is impact. These particles are then forced by the electric field to migrate toward, and collect on, the plates where a dust layer is formed. When the particles contact the grounded plate, they give up electrons, and thus act as part of the collector to future impacting particles.
In dry ESP's, the dust layer is periodically removed from dry ESPs by hammers imparting sharp blows to the edges of the plates 6, typically referred to as “rapping” the plates. Automatic “rapping” systems and hopper evacuation devices remove the collected particulate matter while the ESPs are being used, thereby allowing ESPs to stay in operation for long periods of time.
ESPs perform better if the corona is stronger and covers most of the flow area. This prevents particles that would otherwise flow around the charging zones and escape being charged, which is called “sneakage”. Discharge electrodes have been developed that include rigid structures to which many sharpened spikes are attached, maximizing corona production.
Conventional discharge electrodes are supported on a metal structure, which typically includes a support rod. The rods are conductive in order to electrically connect each spike point with the power supply. Generally, it is considered necessary to have metal spikes that can withstand the electrical currents that often flow due to sparking over between the collection substrate and discharge electrode. Existing discharge electrodes are typically made of metal, which can be quite heavy. In corrosive operating conditions, the charging electrodes are typically made of an expensive alloy (e.g., HASTELLOY brand metal) to avoid or mitigate corrosion in the harsh environments in which such electrodes are used. Since the entire discharge electrode, including the support rod, is commonly made of the same alloy, the electrodes become expensive and heavy, thereby requiring strong support structures.
Two types of electrodes are most commonly used in the industry. The first is an elongated tube with sharp spikes protruding outwardly in different directions using different geometries. The second is a suspended wire electrode that is tensioned by a weight hanging at the bottom of the wire. The existing designs are costly when the discharge environment is corrosive (e.g. in a wet ESP), and the highest discharge current attained by conventional electrodes may not be satisfactory for any environment.
Therefore, the need exists for a discharge electrode that is lightweight and inexpensive, but which has a sufficient current flow to produce high discharge currents and particle collection efficiency along with low susceptibility to corrosion.